1. Field of the Invention
the present invention relates to an improvement in single screw mechanisms of the multiple tooth engagement type for varying the pressure of a fluid such as a liquid pump, gas compressor or expander, rotary engine or the like.
2. Description of the Prior Art
The group of single screw machine which is of concern to the present invention are classified as positive-displacement rotary type machines. Mechanisms of this type generally consist of a mainrotor which meshes with two or more symmetrically opposed gaterotors. The gaterotor teeth sweep through the mainrotor threads drawing fluid into the chamber from an inlet port provided in the machine's housing and forcing fluid from the thread into an outlet port in the housing. Sufficient torque is supplied by prime mover means for rotation of the mainrotor towards the gaterotor tooth to overcome the outlet pressure being generated in the closed pocket of fluid defined in the chamber between the machine housing, mainrotor threads and gaterotor tooth.
In conventional single screw mechanisms, the gaterotor teeth are required to separte the pressure in the closed compression chamber from the inlet pressure of the suction plenum so this is an area that has a large effect on machine performance. Due to the relative angle of motion between the gaterotor and the mainrotor, as the gaterotor's teeth travel through the mainrotor thread, the tooth flanks must be beveled above and below the sealing plane to prevent binding. The angles by which the gaterotor teeth flanks are relieved is determined by the minimum and maximun angles at which the gaterotor tooth edge and the mainrotor thread walls intersect and a radial line from the gaterotor axis to the point of contact. The necessity of providing these reliefs however, presents a problem in that they initiate flow paths out of the chamber enclosed by the tooth along the length of the tooth. As can be seen in FIG. 1, conventional gaterotor tooth design is represented by the convergence of two skewed planes machined at the minimum and maximum relative angles fo motion coming into contact with the mainrotor thread 18 so as to create a single sealing edge along the tooth flank during rotational motion. It should be recongnized that each gaterotor tooth flank must be relieved by these angles and that these flank angles remain substantially parallel about the sealing plane 17 throughout the pressure changing cycle.
Leakage paths out of the pocket in the chamber enclosed in the mainrotor thread between the tooth of one gaterotor and the machine housing can be classified into at least two major kinds. The first of these leakage paths is a route directly past the gaterotor teeth flanks in a direction that is perpendicular to the tooth face. The second major leakage path of concern, is in a direction that is parallel to the tooth face over the crest of the mainrotor thread crest where the root of adjacent gaterotor teeth engage. This latter leakage path will hereinafter be referred to as a blowhole leakage path and occurs at the mainrotor thread crest-gaterotor tooth root-housing interface. Both leakage paths exist in all single screw machinery and it has been demonstrated that the mass of fluid that will flow past the gaterotor tooth from the closed chamber pocket to the suction plenum is proportional to the square root of their absolute pressure difference.
In attempting to limit the amount of leakage past gaterotor teeth, past practice has been to limit the size of the downholes by reducing gaterotor tooth thickness, by reducing pocket sealing plane depth below the pocket side of the gaterotor tooth, and by reducing the difference between the maximum and minimum relief angles requrired at the gaterotor tooth flanks. Several notable attempts have been made in this regard as exemplified, by way of example, in U.S. Pat. No. 3,932,077, wherein volumetric discharge is increased by a gaterotor tooth seal which has arch shaped flanks so that surface to surface contact with the mainrotor thread is made within a zone. The major drawbacks of this design are that mainrotor thread machining takes a long time and that whenever parallel cylinder profiles are used, leakage past the flank areas become major. Recognizing these difficulties, U.S. Pat. No. 4,321,022 proposes gaterotor teeth flanks comprising at least three skewed surfaces which intersect in at least two edges so as to provide dual lines of sealing with the mainrotor thread as can be seen in FIG. 2.
Although many previous single screw pump devices including U.S. Pat. Nos. 1,437,464, 1,654,048, 1,723,157, 2,994,276, and 3,232,236 have utilized a mainrotor which contains thread windings in excess of 180.degree. the inventor's co-pending application Ser. No. 06/908,859, now U.S. Pat. 4,824,348 filed Aug. 25, 1986, which is hereby incorporated by reference, discloses a novel multiple tooth engagement single screw mechanism. This machine improves upon the conventional single screw design by increasing the gear ratio and by allowing the wrap angle to exceed the conventional 360.degree. divided by the number of gaterotors so that several teeth are simultaneously engaged in each mainrotor thread. The advantage of this design is that it breaks down the high to low pressure gradient across two or more gaterotor teeth engaged in a single mainrotor thread. Leakage past the gaterotor tooth flanks in such a mechanism is reduced since the differential pressure across the gaterotor tooth is attenuated.
The previoiusly discussed single screw mechanisms are representative of conventional designs and improvements thereto as disclosed by the inventor and others when dealing with the problem of volumetric efficiency. Although several embodiments have been discussed, the designs of the prior art contain various limitations and offsetting disadvantages so that the effort to reduce leakage remains a major design consideration in single screw machinery. The embodiments hereinafter illustrated and described are distinguishable in the many ways they create better performing seals by reducing or eliminating gaterotor tooth leakage, allowing higher pressure capability, and removing the primary limitation on gaterotor tooth thickness.